Olefinically modified silanes, e.g., vinyltriethoxysilane, 3-(meth)acryloxypropyltrimethoxysilane and the like, are useful as coupling agents in applications where molecular bonding of inorganic substrates and fillers with organic resins and polymers is required. However, these silanes have a tendency to homopolymerize during production, purification and in situ prior to application. Thus, inhibitors are required to prevent such polymerization.
The prior art has taught the use of phenols, such as MEHQ (p-methoxyphenol), and aromatic amines as inhibitors for these polymerizable silanes. See for example U.S. Pat. No. 5,103,032 to Turner et al. However, such inhibitors are disadvantageous in that, at their required levels of use, they absorb ultra-violet light, turn yellow, and undergo photooxidation. In addition, many traditional polymerization inhibitors have relatively high boiling points, restricting their utility as co-boiling inhibitor candidates. Also, many traditional inhibitors, being non-polar, can cause failure of the final silane to pass stringent water solubility requirements--especially at the typical required inhibitor concentrations.
Another problem with phenolic inhibitors is that they require oxygen to work effectively (an amount commensurate with the amount of inhibitor), yet, not too much oxygen because excessive oxygen will form peroxides and thus initiate polymerization reactions. Therefore, phenolic inhibitors require that the level of oxygen be carefully controlled.
Relatedly, the literature also describes the use of piperidinyloxy free radicals as antioxidants for the stabilization of (meth)acrylic acids and esters, as well as other reactive olefin compounds at relatively high levels, 100 to 500 ppm, often in conjunction with a quinone or phenolic co-inhibitor. See for example U.S. Pat. No. 5,254,760 to Winter et al. However, the prior art does not disclose the use of such inhibitors with silanes nor at low levels which would provide the advantages of the present invention.